1. Field of the Invention
The present invention involves a method for determining the ventilation of a lung of an object under examination by means of magnetic resonance imaging.
2. Description of the Prior Art
The process of imaging the lung by means of magnetic resonance imaging basically confronts the user with two problems. The torso of the subject keeps moving because of breathing, with one breathing cycle being about 5 seconds long. Moreover, in addition to the blood vessels, the lungs are basically composed of pulmonary alveoli, and the tissue of an alveolus encloses an air-filled space. This tissue has therein protons and, accordingly can be detected in a magnetic resonance measurement. However, the various air-tissue transitions result in susceptibility jumps (discontinuities) that can cause signal losses. This is especially the case with gradient-echo based methods starting at a field strength of about 1.5 tesla. In order to avoid such artifacts, it is possible to use spin-echo based methods, or the gradient-echo based methods have to be implemented with a minimal echo time.
If several lung-representing image data sets are acquired during one breathing cycle, it is possible by comparing the signal intensities to determine the relative change in ventilation of a specific lung region. This relative change in ventilation is available in the form of a factor by which the ventilation of the image with the lower signal intensity of the lung tissue has been increased in comparison to the image data with a higher signal intensity of the lung tissue. This is the case because by filling the alveoli with air, the tissue portion in an observed volume element is decreased, which reduces in the respective image region the signal intensity with increasing ventilation.
Since only the relative change in ventilation is known, it is difficult to compare the values of different patients.
Spirometry is another known method for determining ventilation parameters. To this end, a spirometer is used to record the change in the lung volume as well as the time course of the change. However, a spirometer can determine only the changes in relation to the lung as a whole.